Originally Posted by
A Squared
OK, fine, the airliner is flying west, so west is the positive direction of our frame of reference. The velocity of the westbound airliner is +500 knots. The man is walking walking in the east direction , so that's -1 knot. a positive 500 knots, plus a negative 1 knot equals positive 499 knots, the man is still moving west at 499 knots. it doesn't become negative because of the direction he is facing.
Oh Christ. You're anchoring the +/- to the geographical direction. I was leaving them anchored to the body. Yes I can belabor the point and rework everything to keep to a geographical frame, but the whole purpose of the analogy was to relate it to what we all intuitively understand from flying bug smashers at 100 knots in 10 knot winds (which is where a positive sign on groundspeed always means that ground is moving opposite the direction the nose is pointing). Look at scenario A!